In our first article in this series: Basics: Offline Maps and How to Get Started Part 1, we provided background on offline maps. We discussed the difference between base-maps and layers, and covered what are interactive and editable map layers. In this second post in the series we will discuss how we take base-maps and layers offline.
1. Taking Base-Maps Offline
For this conversation we will focus much of our attention on ArcGIS. Note, the same basic principles discussed here will apply to other map and GIS offerings. So what is a base-map? It is a series of pictures or tiles combined into a single image. Rather like a jigsaw puzzle. And just like a jigsaw puzzle a single image is usually broken into smaller pieces. Think of a huge satellite image. Take a cookie cutter and break that up into smaller chunks. These are tiles.
So why generate tiles? Improved user experience that is why. Google in 2005 released Google Maps. Suddenly we could pan and zoom satellite images for free on the web. And yes the first thing I did was zoom in to see my house. But take a closer look at Google Maps or an ArcGIS web map today. You will see the individual base-map tiles as you pan and zoom. Your experience is seamless and super fast. Imagine each time you panned the map, just for a moment the map disappeared, then reappeared with the new extent. That was the pre Google Maps reality.
So how does this relate to offline? To be able to use base-maps in areas without wireless connectivity we need to copy these tiles to our smartphone or tablet. In Esri-land this can be done dynamically (see Collector for ArcGIS and the base-map selection option for offline) or through a tile package or TPK. This is simply a zip file of images. Tiles at different zoom levels: a “pyramid” of images.
Why tiled maps? from Penn State is a good article on this subject, well worth reading.